<!DOCTYPE html>
<html lang="en">

<head>
    <meta charset="UTF-8">
    <meta name="viewport" content="width=device-width, initial-scale=1.0">
    <title>京东三角</title>
</head>
<style>
    .jd {
        position: relative;
        width: 200px;
        height: 200px;
        margin: 200px auto;
        background-color: skyblue;
    }
    .jd span{
        position: absolute;
        right: 50%;
        top: -20px;
        width: 0;
        height: 0;
        /* 为了照顾兼容性 */
        line-height: 0;
        font-size: 0;
        border: 10px solid transparent ;
        border-bottom-color: skyblue;
    }
</style>

<body>
    <div class="jd">
        <span></span>
    </div>
</body>

</html>